A specific protocol has been developed for allowing multiple users to transmit over a shared radio channel. For example, the IEEE (Institute of Electrical and Electronics Engineers) 802.11 Standard generally supports access to radio channels based on a method known as Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
In simple terms, this method is based on a xe2x80x9clisten before talkxe2x80x9d scheme. A transmitter device monitors traffic on a shared radio channel to determine if another transmitter device is presently transmitting on the same channel. If the radio channel is in use, the transmitter device will continue to monitor the channel until it is clear. When the radio channel is finally clear, the transmitter will then transmit over the radio channel.
Ideally, another transmitter device will not simultaneously transmit during the same time. However, a collision can occur on the radio channel when two or more transmitter devices transmit on the radio channel simultaneously. Consequently, neither message transmission would be intelligible and both transmitter devices must re-transmit their messages again to a corresponding target device.
Based on this CSMA/CA scheme, re-transmission of data due to a collision cannot occur before a minimum time gap. After the minimum time gap has passed, the transmitter device selects a random xe2x80x9cbackoff interval,xe2x80x9d which is the wait time before the radio channel is again monitored to determine whether the radio channel is clear to transmit. If the channel is still busy, another shorter xe2x80x9cbackoff interval,xe2x80x9d is selected for a subsequent message transmission. This process is repeated until the transmitter device is able to transmit data.
Another standard for transmitting data on a shared radio channel is based on IS-95, in which multiple field units can transmit at the same time.
The IS-95 standard suggests a method of ramping RF power of a field unit until a message from the field unit is transmitted at a power level that is detectable at a base station. According to this method, a field unit transmits an access request message to a base station for the assignment of wireless resources on a reverse link.
After transmitting an access request message on an access channel, the field unit monitors a paging channel for an acknowledgment message from the base station indicating that the access request message was properly received. If no acknowledgment message is sent to the requesting field unit, it is presumed that the message from the field unit was not transmitted at an appropriate power level. That is, the power output level of the field unit is so low that the base station did not detect a previously transmitted access request message. The access request message is then re-transmitted over the access channel at a higher power level.
This process is subsequently repeated until the field unit transmits a message at a power level that is high enough for the base station to properly receive the message. Similar to the IEEE 802.11 standard, a collision can occur on the shared radio channel when two or more field units simultaneously transmit a message.
The present invention is generally directed towards an apparatus and method for enhancing the utilization of resources in a wireless communication system. In an illustrative embodiment, messages are transmitted over a shared channel to a target receiver. The shared channel is monitored for collisions that can occur when two or more transmitter devices attempt to send a message at or about the same time. Feedback is provided to notify one or multiple transmitters when a collision is detected.
One method of notifying the transmitter devices of a collision is to provide feedback in a reverse channel to the transmitter devices. More specifically, a device such as a base station monitoring the shared channel for collisions can transmit messages indicating that a collision occurred on the first channel for a previous message transmission.
In a specific application, the transmitter device is a field unit transmitting a message such as an access request message to a target receiver monitoring a channel for messages. For example, the shared channel can be a reverse link radio channel defined by codes such as those used in wireless CDMA (Code Division Multiple Access) communication systems. More specifically, the shared reverse link channel can be an access channel. The second channel for notifying field units of message collisions can be a forward link radio channel defined by another unique PN (pseudnoise) code.
To support communications between a transmitter and target receiver device such as a base station, the transmitter can synchronize itself prior to transmitting messages on the shared channel. Thus, there can be at least some structure as to when a transmitter device sends a message over the shared channel. Alternatively, a transmitter device can send a message asynchronously over the shared channel.
In an application where the transmitter is at least grossly synchronized with the target receiver, the transmitter can then transmit in a time slot or data field of the shared channel. Consequently, a device monitoring the shared channel for message collisions can monitor time slots of the shared channel for messages. One way to determine whether a message is properly received is to provide redundancy information in the message itself, where a check can be performed to verify that a message is properly received.
If a transmitter device sends a message over the shared channel and no collision is detected at the target receiver, a message such as an ACK (Acknowledgment) message can be fed back to the transmitter device indicating that a message from the transmitter was successfully received at the target receiver without detecting a collision. This ACK message is optionally fed back to a transmitter device over a third channel such as a paging channel of a CDMA communication system. Thereafter, a more formal communication link is optionally established between the transmitter and target receiver for future communications.
A more formal communication link can include a feedback loop for synchronizing a transmitter device to a target receiver when no data payload is being transmitted from the transmitter device target receiver. For example, a target receiver can analyze received messages and generate power and timing adjustment messages on a feedback channel to provide synchronization and power control. Since the feedback loop can be used to provide more sophisticated synchronization between two communication devices, wireless resources can be more quicky allocated for on-demand data payload transfers.
In one application, the second channel or feedback channel is time-slotted or partitioned so that transmitter devices can be notified via feedback messages in time slots similar to the first channel. Consequently, each of the multiple transmitters can monitor designated time slots of the second channel for feedback messages.
A feedback message can be as simple as a single bit indicating whether or not a collision occurred on the shared channel for a previous access request message transmission. Likewise, a sequence of bit information or multiple spaced bits can also be sent over the second channel indicating that a collision occurred. When used, multiple bits can provide redundancy to some extent so that a message can still be conveyed even if part of a message is otherwise corrupted due to a failed data transfer.
In an application where the shared channel is time-slotted, each of the multiple transmitter devices is preferably synchronized so that it can send a message to a target receiver in generally any time slot. When transmitter devices sporadically transmit messages in a time slot to a target receiver, there is an increased likelihood that a collision will occur with another transmitter sending a message in the same time slot.
Although a message format can vary depending on a particular application, one embodiment of the present invention involves sending a two-part message on the shared channel. For example, a first portion of a message commonly used by a set of multiple transmitter devices can indicate a message type such as an access request message. More specifically, the first portion of a message can include a specified sequence of bits or information that identify a message type at the target receiver. Further, a second portion of a message can identify unique information such as the serial number of the transmitter sending the message to the target receiver.
When a two-part message is simultaneously transmitted to a target receiver from two or more transmitter devices, a collision can occur. One way of detecting a collision is to monitor a time slot of a first channel on which multiple transmitter devices may transmit a message. A first portion of each message will preferably line up with each other when transmitted by two transmitters in a time slot so that a monitoring target receiver can identify that a message such as an access request message was sent by at least one transmitter. More specifically, the monitoring device can decode and identify a message type based upon an overlapping bit sequence transmitted by two or more transmitters in a same time slot. The monitoring device can also identify when only one message is transmitted in a time slot by analyzing a unique portion of a message.
Consider further the second portion of a message transmitted by two or more transmitter devices. The second portion of a message transmission is preferably unique so that a collision can be detected at a monitoring device. That is, the second portion of each message can be unique so that the data does not necessarily overlap with each other when two or more transmitters send a message in a time slot. Thus, when a collision occurs, the monitoring device cannot properly decode either unique second portion of a message transmission in a time slot.
One way to identify a message collision is to detect when the second portion of a message as received at a monitoring device does not include proper CRC (Cyclical Redundancy Check) information. For example, a monitoring device can identify a bad CRC check result for a second portion of a received message in a time slot. Although the first portion of the message is properly received and indicates that at least one transmitter is sending a message, processing of the second portion of the message at the monitoring device can indicate that a collision occurs when an error is detected. Consequently, the device monitoring the shared channel for collisions can generate a message over the second channel to the transmitter indicating that a collision occurred at the monitoring target receiver in a given time slot.
In the event that both a first and second portion of the message can be verified as properly received, the monitoring device can determine that a collision did not occur by two or more transmitter devices transmitting in the same time slot and identify which transmitter sent the message. Thereafter, an ACK message can be sent to the appropriate transmitter device to establish a communication link between a requesting field unit and base station.
If used, error detection and correction information can be included in both the first and second part of a message to determine whether there was a collision of either the first or second portion of a message. One of multiple codes in the first portion of the message can be used to identify a message type.
Another aspect of the present invention involves setting a transmitter power level so that an initial message transmission minimally interferes with other transmitter devices. In such a case, an initial power level can be set so low that it may not be detected by a target receiver monitoring the shared channel.
A power output level of a transmitter device can be adjusted depending upon whether a message collision is detected. More specifically, if no collision is detected and no ACK message is sent to a transmitter indicating that a message was received, the power output level of the transmitter can be increased for subsequent message transmissions. Typically, the power output of a transmitter device is increased a predetermined amount for each subsequent attempt to transmit a message to a target receiver if there is no collision. For example, if a message is not acknowledged by the target receiver monitoring the first channel, the power output level for a subsequent message transmission can be increased by 0.5 db (decibels) to increase the likelihood that it will be detected.
If a message from a transmitter is properly received at a target receiver, this power output level of the transmitter is used to determine at what power level subsequent messages can be transmitted for other modulation rates. Power level adjustments for subsequent message transmissions can also be tracked by a transmitter for determining at what level the target receiver acknowledges receipt of a message from a transmitter device.
Yet another aspect of the present invention involves re-transmitting a subsequent message based on a random back-off time. For example, if a message collision occurs, a subsequent message can be transmitted at a random future time. Consequently, two transmitter devices that previously experienced message collisions in a specific time slot can subsequently re-transmit in preferably different time slots so that their corresponding subsequent message transmissions can be properly detected at a device monitoring the first channel. When a message is re-transmitted due to a message collision, the subsequent message re-transmission can be transmitted at a previous power level since a collision was detected and it is not known whether the monitoring device would have otherwise properly received a message if the collision did not occur.
Another embodiment of the present invention involves detecting that a collision occurs when more than two transmitter devices attempt to send a message over a first channel. A power output level of a corresponding transmitter is then adjusted depending upon whether a collision occurs. Consequently, the power output level of a transmitter is adjusted higher so that subsequent message transmissions can be detected at a target receiver monitoring the first channel for message collisions.
In the event that no collision is detected, the power output level of the transmitter can be increased a predetermined amount such as 1 dB. If no collision is detected for a previous message transmission, a transmitter can re-transmit a message at a previous power level.
A transmitter is optionally notified of collisions detected on the first channel based on a feedback message received on a second channel. The feedback messages on the second channel can be transmitted by a device monitoring the first channel for message collisions. A feedback message can include information such as a single bit, a bit sequence or spaced sequence of bits indicating whether a collision was previously detected at the monitoring device.
Another embodiment of the present invention involves allocating a first channel on which two or more transmitters may attempt to transmit a message. The first channel is divided into time slots in which one or more transmitters may attempt to send a message. A first part of the message typically includes a common coded sequence that overlaps when two or more transmitters send a same message in a time slot. Since simultaneously transmitted messages overlap in at least some respect for a particular message type, a device monitoring the first channel can identify a common message type transmitted by the two or more transmitters. Consequently, a monitoring device is able to identify that a message was sent by one or more transmitter devices.
A second part of a potentially two-part message can be unique to a corresponding transmitter. In one instance, the second part of a message can be used to identify whether a collision occurs between two transmitter devices. For example, the first part of the message received at a target device can indicate that a least one transmitter device sends a message since a portion of the message transmissions by two or more simultaneously transmitting devices can overlap. The second part of the message if properly received, can indicate that no collision occurred and only one transmitter sends a message in a time slot. For example, if the second part of a message is not properly received at a target receiver, it can be determined that a collision occurred by two or more transmitters transmitting in a time slot.
In one application, a redundancy check sequence is included in the second portion of a message transmission so that a monitoring device can determine if a message is properly received.
The first part of a message can also include redundancy check information for determining whether multiple transmitters send a common message within a time slot. For example, two different transmitters may send two different message types within a same time slot and, thus, no portion of a message might overlap in such a case.
In one application, the second part of a message includes information indicating from which transmitter a message is transmitted. Consequently, a monitoring device can reply by sending a message directed specifically to the transmitter.
A second channel can also be used to provide feedback messages to the transmitter devices to indicate whether a collision is detected at the device monitoring the first channel.
As previously discussed, certain aspects of the present invention can be used to support access of wireless communication resources. For example, multiple transmitter devices attempting to simultaneously transmit messages on an access channel can be monitored for collisions. The collision information can then be used for multiple purposes. For example, a transmitter device can determine whether or not a previous message transmission is even detected a target receiver. If not, the message can be re-transmitted to the target receiver until a reply is received.
Collision feedback information can also be used to adjust a power level of a transmitter device. For example, if a collision is not detected or an acknowledgment message is not received on a feedback channel, a corresponding transmitter device can subsequently transmit a message at a higher power output level to increase the likelihood that a message is detected at a target receiver. Accordingly, aspects of the present invention minimize interference on radio channels as power level outputs of the transmitter devices are reduced. More specifically, a transmitter device eventually sends a message at a minimal, but detectable power level.
The average time necessary to transmit a message to a target receiver is also reduced because a transmitter is notified of a collision in a feedback path and can therefore re-transmit a message in a random future time slot, reducing the likelihood of a subsequent collision.